All these things depend upon the levels of preparation undertaken. By organizing ahead of time the delays to get food production back into full swing would be minimal.

If you build en masse in major metropolitan and rural areas:

A. Sufficient sheltered underground gardens similar as planned for the moon or mars, that are watered by their own system and safe from contamination.

B. Create 3 official shortwave CD radio frequencies and provide transceivers that receive and broadcast only on these 3 bands. Equip all shelters with 3 transceivers. In this way all shelters can monitor all frequencies at once and would have two back ups in case the others were destroyed. Establish one frequency for search and rescue, one frequency for food and supply needs, another for medical emergency communication. Provide each shelter with a fuel generator and fuel for two weeks operation and possibly a solar panel that can be placed into operation for building up electricity when the fuel runs out.

C. Provide all shelters with radiation and chemical air filtration systems and adequate ventilation for their intended capacity.

D. Provide and store sufficient medical supplies, including various antibiotics, bandages, etcetera.

By planning ahead even recovering the food supply so that many do not starve will help restart civilization. One wont save everyone, even if one really just saves 5% of the population from whatever major nuclear disaster befalls the nation - one will have saved the lives of 15 million people or about the population the US had back in 1840. That's a heckuva lot better than just having a 30 million survive and then killing eachother off or succumbing to starvation, radiation and disease and dwindling to 100,000 or the population of the colonies back around 1690.

Civil Defense now. Homeland Security has become too political and I would even support a private industrial effort to secure these things, since bureaucrats are largely inept and slow responders. Heck, I would donate a few days just to research and prepare an overview of the needs and how they could best be addressed by a CD preparation plan.